I've faced with this problem: The WFS POST request that OpenLayers
performs does not contains following namespaces:
xmlns:wfs="hxxp://www.opengis.net/wfs"
xmlns:ogc="hxxp://www.opengis.net/ogc"
xmlns:gml="hxxp://www.opengis.net/gml"
xmlns:xsi="hxxp://www.w3.org/2001/XMLSchema-instance"

The weird thing is that these namespaces are mentioned in test
expected value at OpenLayers\tests\Format\WFST\v1_1_0.html

...

STATE_NAME
STATE_FIPS
STATE_ABBR
-->

...

I have analysed this test with FireBug and get following:

expected value (exp) is:

STATE_NAME
STATE_FIPS
STATE_ABBR

And real value (got) is:

STATE_NAME
STATE_FIPS
STATE_ABBR

As you can see there is no topp, wfs, ogc, xsi namespaces but test is
resulted as PASSED.

The fact that mentioned namespaces are missed caused following problem
with the Geoserver: